The present invention relates to satellite-borne instrumentation for low-energy charged particle measurements; and more particularly, it relates to the maintenance of a stable sensor potential near the ionospheric plasma potential.
A problem that confronts instrumenters attempting to make in situ low-energy charged particle measurements with satellite-borne instruments is that of maintaining a stable sensor potential near the ionospheric plasma potential. The problem is primarily due to electron currents collected by positive potentials exposed to the plasma on the satellite solar arrays. Past experience has shown that spacecraft whose solar panels are operated with their positive terminals grounded, thus exposing a negative solar array potential, are characterized by having a steady and slightly negative vehicle potential that is ideal for making these measurements. The NASA Atmosphere Explorer and Dynamics Explorer spacecraft utilized positive ground systems and demonstrated these characteristics. Conversely, spacecraft with negative grounds tend to have rather large and erratic negative potentials. The NASA OGO 6 satellite, for example, had a variable vehicle potential some 20 V negative when the solar array was in the sunlight.
Factors such as cost, the use of off-the-shelf hardware, and reliance on past successful designs have resulted in the incorporation of the negative ground systems in most satellites flown by both NASA and the Department of Defense, and efforts to conceal the positive solar array potentials from the plasma have mostly been either nonexistent or costly and unreliable. Since the space shuttle uses a +28 V system (negative ground), the advent of the shuttle-launched satellite era may well result in the exclusive use of negative ground systems in future satellite designs to assure compatibility with the shuttle while the satellite is in the bay.
Low-energy particle detectors can in fact be designed to operate on satellites with large negative vehicle potentials by electrically isolating the sensor system and biasing it to some potential. Electrical isolation of the sensor system is relatively straightforward in that mechanical elements can be isolated with conventional insulators and the electronics can be isolated by using floating power suppies with transformer isolation and optical couplers or level shifter circuits for data and timing signal interface with the spacecraft. Heretofore, however, no straightforward method for maintaining a bias potential near the plasma potential has been demonstrated.